During any fatigue test, loads of controlled magnitudes are applied to a test specimen. The loads may all be the same or they may be of differing amplitudes. At some time during such a test, a fatigue crack will initiate in the specimen and subsequently grow. The larger the fatigue crack becomes the lower the residual strength of the specimen becomes. Finally, the residual strength will be less than that required to resist the current test load, and the specimen will break. The load that causes a specimen to break is generally less than the load control device is attempting to apply to the specimen; but the exact value of the breaking load is unknown.
In a material fatigue test the load usually recorded is the one which the test machine is attempting to apply to the specimen not the actual load at which failure occurs. Hence, this measurement is not the actual failure load. The actual failure load can be determined, but this requires an oscillograph or similar equipment to make continuous load recordings. However, measurements of this type are not practical because of the expense and volume of oscillograph or other recording paper.
It is therefore the primary purpose of this invention to provide a simple and economical means for measuring the load on a test specimen at the instant failure occurs in the specimen.